The continual demand for enhanced speed, capacity and efficiency has resulted in dramatic advances in a variety of manufacturing fields (e.g., electronics, communications, and machinery). Among many recent developments, the field of electro-mechanics has focused significant attention on the miniaturization of various devices. A micro-electromechanical system (MEMS) is a system that usually has electrically controllable micro-machines (such as a motor, actuator, optical modulating element, etc.)—most often formed monolithically on a semiconductor substrate using integrated circuit techniques.
Several micro-actuator technologies have been investigated for positioning individual elements in MEMS applications. Electrostatic, magneto-static, piezoelectric and thermal-expansion systems have been used in varying degrees for micro-actuator operation. From this field of technology, asymmetric electro-thermal actuators have proven particularly useful in a number of MEMS applications.
Generally, a MEMS polysilicon surface micromachined electro-thermal actuator uses differential heating to generate thermal expansion and movement. In one conventional asymmetric electro-thermal actuator design, a single “hot arm” is narrower than a “cold arm.” When electric current is applied, the electrical resistance of the hot arm is greater. When an electrical current passes through both the hot and cold arms, the hot arm is heated to a higher temperature than the cold arm. This temperature differential causes the hot arm to expand along its length, thus forcing the tip of the actuator to rotate about the flexure. Another variant of the asymmetric design joins together arms of similar size and shape, but having substantially different coefficients of thermal expansion. In such design, a “hot arm” has a higher coefficient of thermal expansion than that of a “cold arm.” When electric current passes through both the hot and cold arms, the hot arm expands more than the cold arm, effecting the desired actuator movement.
Frequently, electro-thermal actuators are deployed as bi-stable switches—i.e., as elements that switch between a first position, when no current is applied, and a second position, when current is applied. Once the current is removed, the actuator returns to its initial position.
Such MEMS components may be utilized in a wide variety of electrical or mechanical switching applications. Application of MEMS-scale switches may be of particular use in the wireless communications field; especially as portable wireless communication devices continue to strive for greater performance from devices of decreasing form factor.
There are a number of potential switching applications in wireless communication products that could benefit from MEMS-scale components. Consider, for example, a multi-mode cell phone. It would be advantageous, from a size and form factor perspective, to use a MEMS switch to shift operation between modes. In these and other small, battery-powered wireless communications devices, however, power consumption must be also minimized in order to extend time of operation on a battery charge. As such, a conventional MEMS-scale switch component may be of limited utility in such devices—since such switches often return to a default position in the absence of power. Another consideration is that a number of applications may require a multi-position switch, having more than just two positions or states. Implementing such applications using only bi-stable MEMS switches would either be cumbersome or infeasible.
Furthermore, even where a conventional bi-stable MEMS switch may be suitable, the direct interface between semiconductor circuitry and operational MEMS structures can still cause operational problems. For example, operation may require deployment of a conventional MEMS switch while a second, adjacent operational element is electrostatically actuated. Given the minute scale of such structures and the separations therebetween, the electrostatic signals actuating the second element could adversely affect the MEMS switch, leading to a malfunction or performance loss. Brute force solutions, such as complex routing layouts, might be employed to overcome such a problem, but they also introduce a number of inefficiencies to device manufacturing or operation.
As a result, there is a need for a system that provides reliable and sustainable MEMS switching, without relying on continuous electrostatic or electromagnetic force—one that is readily adaptable to a number of production or manufacturing processes, and to address a variety of specific design requirements, including the provision of multi-throw switches—while providing reliable device performance in an easy, efficient and cost-effective manner.